Recently, for print images, the dpi of printing apparatuses such as a printer which prints digital images is increasing along with the spread of personal computers, digital cameras, and the like. Of printing apparatuses, ink-jet printing apparatuses using an ink-jet printing head rapidly prevail. Demands arise for lower-cost ink-jet printing apparatuses which realize high-resolution images.
A serial scanning ink-jet printing apparatus prints an image by one scanning at the printing head width while scanning the printing head in a direction (main scanning direction) perpendicular to the convey direction (sub-scanning direction) of a printing medium such as a printing sheet. At the end of printing by one scanning, the ink-jet printing apparatus conveys the printing medium by the printing head width in the convey direction. The ink-jet printing apparatus sequentially repeats image printing by one scanning and conveyance of the printing medium described above, forming a desired image on the printing medium.
In a conventional printing apparatus, data sent from a host computer are data sequential in the main scanning direction (to be referred to as raster data hereinafter). To print an image by one scanning at the printing head width, data must be converted into data sequential in the sub-scanning direction (to be referred to as column data hereinafter) in accordance with the number of print elements (e.g., the number of ink discharge nozzles) of the printing head. To print an image by one scanning, the printing apparatus must hold at least a memory area for accumulating raster data by the number of discharge nozzles and a memory area for accumulating column data.
A higher image quality is required by increasing the dpi of the printing apparatus. A higher dpi of the printing apparatus increases the image resolution, resulting in a large data amount for image printing. The memory area necessary for the printing apparatus also increases, and an expensive large-capacity memory leads to high cost, inhibiting cost reduction of the printing apparatus.
A technique of reducing the memory area necessary for the printing apparatus is disclosed in Japanese Patent Laid-Open No. 11-259248. According to Japanese Patent Laid-Open No. 11-259248, isochronous data transfer is performed between a host computer and a printer by minimum buffering. This can reduce the memory capacity necessary for the printing apparatus.
More specifically, image data of one scanning is divided into data frames and transferred in isochronous data transfer between the host computer and the printer. When the printer receives image data transferred from the host computer, the printer processes the image data and temporarily holds it in the memory area of the printer. This memory area stores one or more divided data frames of image data during printing operation by one scanning.
In isochronous data transfer in which image data of one scanning is divided into data frames and transferred, data frames of image data of one scanning are successively transmitted to the memory area of the printer. The memory area size of the printer suffices to ensure at least a memory area capable of storing one or more data frames, and the memory area need not store image data of one scanning. Unlike a conventional printer, a memory area for storing image data of one scanning is not required.
In the above-described isochronous data transfer, printing operation by one scanning can start upon reception of minimum data necessary for image printing without storing image data of one scanning in the memory area of the printing apparatus. Sequentially transmitted data frames are stored in the memory area of the printer while the memory area is rewritten. An image by one scanning can be formed without interrupting scanning of the printing head, while the memory capacity necessary for the printing apparatus can be reduced.
In the above-described isochronous data transfer, a job and task in the host computer cannot be managed by the printing apparatus. Data transfer from the host computer to the printing apparatus may be left undone due to any reason.
If image printing is interrupted during one scanning, as described above, a difference (printing time difference) in the printing time on a printing medium is generated between a dot at the interrupted position and a dot at a restart position adjacent to the interrupted position. It is generally known that the density change caused by the printing time difference appears as density unevenness (printing time difference unevenness) in a printed image in the presence of the printing time difference. The reason is as follows.
Ink droplets landed on a printing medium permeate in a direction perpendicular to the printing medium (e.g., paper) (direction of thickness of the printing medium) and a direction in which ink droplets spread on the surface of the printing medium. A pigment such as a dye serving as an ink component is physically and chemically bonded with the printing medium.
If a time difference (t2−t1) between printing times (t1 and t2) when two adjacent dots are printed is small, ink droplets landed on the printing medium later (t2) also permeate in the direction perpendicular to the printing medium and the direction in which ink droplets spread on the surface of the printing medium. However, these ink droplets hardly permeate and fix in a region where ink droplets landed earlier (t1) fix.
This is because ink droplets landed earlier (t1) are permeating yet, and the printing medium and ink component can be chemically bonded with each other by a limited amount. Thus, ink droplets landed later (t2) permeate and fix below the region where ink droplets landed earlier (t1) permeate.
If the time difference (t2−t1) between the printing times (t1 and t2) when two adjacent dots are printed is large, ink droplets landed later (t2) permeate by a larger amount than ink droplets for the small time difference (t2−t1) in the region where ink droplets landed earlier (t1) permeate and fix.
This is because ink droplets landed earlier (t1) sufficiently permeate and spread, or the volatile component of ink droplets evaporates, the ink droplet amount per unit area decreases, and ink droplets landed later (t2) spread and penetrate to the region of ink droplets landed earlier (t1).
More specifically, if the time difference between printing times when adjacent dots are printed is large, ink, i.e., a pigment or ink component such as a dye fixed near the surface of the printing medium remains in a large amount. The density is related to light absorption of the pigment fixed near the surface of the printing medium, and a large printing time difference leads to a high density. If printing operation is interrupted during one scanning, density unevenness occurs in images printed before and after interruption, and the boundary between the images before and after interruption appears as a stripe, greatly degrading the image quality.